Our laboratory has adapted a primary culture system of cells dissociated from embryonic mouse brain to a form suitable for studying myelination in vitro. Furthermore the culture system responds to thyroid hormone in a manner similar to the way brain does in the intact animal. This system will be used to study the mechanism of regulation of myelination by thyroid hormone. The system enables us to delve within the cell to study the regulation of myelination at a level of sophistication not previously possible. Other effectors of myelination will also be investigated.